1. Field of the Invention
The present invention relates to resistive touch screens, and more particularly, the use of a resistive touch screen as a proximity sensor.
2. Description of Related Art
Resistive touch screens are well known in the art and are used by electronic devices as inputs to detect physical touches by the user. The most common resistive touch screens are the 4-wire resistive touch screen and 5-wire resistive touch screens.
A 4-wire resistive touch screen comprises multiple resistive layers that are separated by thin spaces. When a user touches a screen with a finger or stylus, physical pressure is applied to the layers, which in turn, causes the layers to touch at the point where the pressure is being applied. To determine the exact location of the touch, a voltage gradient is first applied horizontally to the top resistive layer. The bottom resistive layer is used to measure the voltage level of the top resistive layer at the point of touch between the two resistive layers. The horizontal coordinate is determined based on the voltage level at the point of touch. For example, if a voltage gradient of 0 to 5 volts were applied horizontally to the top resistive layer such that one end of the top resistive layer is at 0 volts and the other end is at 5 volts, a measurement of 2.5 volts would indicate the touch point to be in the middle of the horizontal axis. A voltage gradient is then applied vertically to the bottom resistive layer and the top resistive layer is used to measure the voltage at the touch point, thus providing the vertical coordinate.
A 5-wire resistive touch screen also contains two layers. The top layer is a low impedance conductive layer and the bottom layer is a resistive layer. The bottom layer contains four electrical conductors, one at each corner. When the screen is touched, the top layer makes contact with the bottom layer. To determine the horizontal coordinate of the touch, a voltage gradient is applied to the bottom layer using the two electrical conductors located at the left or right side of the bottom layer. The top conductor layer is then used to measure the voltage level at the point of the touch. Similar to the 4-wire resistive touch screen, the horizontal coordinate is determined based on the voltage level at the point of the touch. To determine the vertical coordinate of the touch, a voltage gradient is applied to the bottom layer using the two electrical conductors located at the top or bottom side of the bottom layer. The top conductor layer is then used to measure the voltage level at the point of the touch and the vertical coordinate is determined based on the voltage level.
Both 4-wire and 5-wire resistive touch screens typically have a large capacitance between the top and bottom layers because of the close proximity between the two layers.
Resistive touch screens are often used in mobile devices that have both a touch screen and a liquid crystal display (LCD) or similar type of display. Tablet computers and smartphones are some examples of such devices. Due to the mobile nature of the devices, power saving is an important consideration, and individual components should be deactivated when not in use. For example, when a user is talking on the phone, the display is not required to be on. Thus, the ability for the phone to sense when a user is holding the phone close to, or adjacent to the ear, would allow the designer to save power by turning off the display during that time.
FIG. 1 depicts a simplified circuit diagram of a system that is typical of the prior art. The system 101 contains two separate and distinct components: a photodiode-based proximity sensor component 103 to detect the proximity of the user, and a resistive-touch-screen component 105 to detect user inputs. The proximity sensor component 103 contains an infrared photoemitter (IRLED) driver 109, IRLED 115, photodiode 111, and photodiode sensor 107. When an object approaches system 101, the amount of light that photodiode 111 is exposed to changes, resulting in a change in current level generated from photodiode 111. Photodiode sensor 107 detects the current change, allowing the system to react to the approach of the object accordingly. The resistive touch screen component 105 contains resistive control circuit 123, resistive sensor 121, top resistive layer 117 and bottom resistive layer 119 of the resistive screen. When a user presses down on top resistive layer 117, it makes contact with bottom resistive layer 119 at the coordinate where the force is applied. Resistive control circuit 123, using resistive sensor 121, detects the contact between the two layers and determines the location of the contact, allowing the system to react to the user touch accordingly.
The typical prior art system depicted in FIG. 1 contains both a proximity sensor and a resistive touch screen, but the solution is not efficient nor cost effective because it requires two distinct circuits. Accordingly, being able to use the resistive touch screen as a proximity sensor reduces the size and cost of the system, and does not add any additional components or chassis modification.